Semiconductor light emitting devices such as light emitting diodes (LEDs) are among the most efficient light sources currently available. Material systems currently of interest in the manufacture of high brightness LEDs capable of operation across the visible spectrum include group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials; and binary, ternary, and quaternary alloys of gallium, aluminum, indium, and phosphorus, also referred to as III-phosphide materials. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates and III-phosphide devices are epitaxially grown on gallium arsenide by metal organic chemical vapor deposition (MOCVD) molecular beam epitaxy (MBE) or other epitaxial techniques. Often, an n-type layer (or layers) is deposited on the substrate, then an active region is deposited on the n-type layers, then a p-type layer (or layers) is deposited on the active region. The order of the layers may be reversed such that the p-type layers are adjacent to the substrate.
Some of these substrates are insulating or poorly conducting. In some instances, a window is attached to the semiconductor layers to enhance optical extraction. Devices fabricated from semiconductor crystals grown on or affixed to poorly conducting substrates must have both the positive and the negative polarity electrical contacts to the epitaxially grown semiconductor on the same side of the device. In contrast, semiconductor devices grown on conducting substrates can be fabricated such that one electrical contact is formed on the epitaxially grown material and the other electrical contact is formed on the substrate. However, devices fabricated on conducting substrates may also be designed to have both contacts on the same side of the device on which the epitaxial material is grown so as to improve light extraction from the LED chip. There are two types of devices with both the p- and n-contacts formed on the same side. In the first (also known as the flip chip), the light is extracted through the substrate or window material. In the second (also known as an epi-up structure), the light is extracted through the contacts, through the uppermost semiconductor layers of the device, or through the edges of the devices.